1. A sound of annoyance, often used to segway into a conversation about why one may be so annoyed.
2. A filler word, used to imply "I'm so bored, but I don't know what to say to start a new conversation."
3. To be shutdown.
4. Implies boredom/confusion with a conversation, so much that one is too lazy to debate it.

A: Mreh.
B: What's wrong?

A: ...and that is how I solved the dilemma of "how to clean my bathroom sink"
B: Mreh.